1. Field of the Invention
The present invention generally relates to a semiconductor device in which electronic components such as semiconductor chips are supported by a wiring board, and more particularly to a semiconductor device having projection electrodes such as electrodes of a ball grid array type (hereinafter simply referred to as BGA type).
Recently, the integration density of semiconductor chips has been drastically increased and it has been required to increase the mounting density of semiconductor devices.
Under the above situation, there has been considerable activity in the development of BGA type semiconductor devices, which has advantages in which electrodes (terminals) for external connections can be arranged on the back surfaces of the semiconductor devices at a relatively wide pitch and are hardly deformed. These advantages are attractive as compared to QFP type semiconductor devices. As the integration density of the semiconductor chips is increased, it is necessary, at the initial stage of the production of the semiconductor devices, to cope with various situations such as modifications of the functions of the semiconductor devices and unexpected problems thereof. Generally, patterns of wiring lines formed on a mother board are altered in order to cope with various situations as described above. It will be noted that the term "mother board" may be referred to as a printed circuit board, a circuit board, a wiring board, a mounting board or just board.
As a practical problem, it takes much time and a large amount of work to alter the patterns of wiring lines on the mother board. Hence, as a simple and easy way, a strap line intended to shortcircuit patterned lines on the mother board is used to achieve the necessary alteration of the circuit. Such a strap line is used on the main (mounting) surface of the mother board on which the semiconductor device is to be mounted. When the semiconductor device has terminals for external connections along edges thereof, it is possible to simply and easily modify the pattern by the strap line. However, the BGA type semiconductor devices do not have such terminals but ball-shaped terminals provided on the bottom surface of the package (more particularly, the bottom surface of the base supporting the semiconductor chip). Hence, it is impossible to extend the strap lines from the ball-shaped terminals on the mounting surface of the mother board.
2. Description of the Related Art
FIG. 1 is a perspective view of a conventional BGA type semiconductor device. A BGA type semiconductor device 10 shown in FIG. 1 is a semiconductor chip (an LSI chip) 14, which is a bare chip, a sealing part 16, and SnPb solder bumps 18 having a ball shape. The device 10 is an example of an electronic component having an integrated circuit. Electrodes (not shown) provided on the semiconductor chip 14 and electrodes provided on the front surface of the wiring board 12 are bonded together by wires. The electrodes provided on the wiring board 12 and the solder bumps 18 provided on the back surface of the wiring board 12 are electrically connected together through wiring layers and via holes formed inside the wiring board 12. The surfaces of the semiconductor chip 14 and the wiring board 12 are sealed by the sealing part 16 made of resin. The solder bumps 18 function as terminals (electrodes) for external connections, and are arranged on the back surface of the wiring board 12 in a matrix formation. Generally, the interval between the adjacent solder bumps 18 is equal to or less than 1.5 mm.
The BGA type semiconductor device 10 can be mounted on a mother board 20, which may be a glass-epoxy resin board. On the mother board 20, there are provided electrodes 22 arranged in a matrix formation and wiring lines. The semiconductor bumps 18 can be soldered to the electrodes 20 on the mother board 20 in a state in which the bumps 18 are in contact with the electrodes 20.
As described above, as the integration density of the semiconductor chip is increased, it is more frequently required to alter the circuit pattern on the mother board in order to cope with modifications of the function of the semiconductor device and problems thereof.
FIG. 3 is a diagram which shows how to provide a strap line (which is also called a jumper line) in order to perform a circuit modification in a state in which a QFP type semiconductor device 24 is mounted on a mother board. A lead terminal 26 of a QFP type semiconductor device is soldered to a land 28 formed on a mother board 20. A reference number 30 indicates a solder filet. It is now assumed that a circuit modification needs to make a connection of a strap line 36 with the lead 26. In this case, a conductor covered by an insulating coat of the strap line 36 is soldered to the land 28 by using a pulse heat tool 32 or a soldering iron 34. As will be seen from the above, it is easy to connect the strap line to the lead which outwardly extends from the edge of the semiconductor device.
However, as shown in FIG. 2, it is impossible to connect the strap line 36 to a terminal of the BGA type semiconductor device 10 (the terminal is already in the filet state) in the state in which the device 10 is already mounted on the mother board 20. Hence, a circuit modification using the strap lines cannot be made for the BGA type semiconductor devices. Hence, it is necessary to directly modify the circuit pattern formed on the mother board.